Sintered materials, such as sintered ceramics, are finding ever widening structural application as parts in machines, engines and other industrial equipment.
Most sintered bodies are prepared by molding powder into a compacted shape which is then fired to sinter the powder. The sintering results in a monolithic body. This is the basic method used to make many structural ceramic bodies.
During sintering, the powder compact is densified (i.e., porosity is reduced) to improve the mechanical integrity and other physical properties of the resultant body. Densification is inherently accompanied by shrinkage of the compacted shape. The amount of shrinkage depends on the amount of density increase which is a function of the firing conditions, the original powder characteristics, and the powder packing. Typical production methods using sintering to achieve low porosity involve linear shrinkages on the order of at least about 15% from the original compacted shape.
Control and prediction of shrinkage are essential for industrial production of useful sintered articles or parts. The final dimensions of a sintered part are determined by the dimensions of the starting powder compact and the subsequent shrinkage of the compact during sintering. The degree of accuracy, uniformity, and repeatability in achievement of the required final sintered part dimensions directly affect the cost of production.
For sintered parts having final sintered dimensions larger than desired, machining to correct for dimensional variations (resulting from inadequate control and/or prediction of shrinkage) adds significantly to the product cost.
A more serious cost effect associated with sintering shrinkage occurs when sintered parts must often be rejected as scrap for failure to maintain the targeted dimensions (e.g. a section that is too narrow or a protrusion that is too short). Rejection for dimensional insufficiency may be caused not only by lack of accuracy of overall shrinkage, but also by serious distortion due to non-uniform shrinkage even if the proper average shrinkage is achieved. Besides causing shape distortion leading to insufficient dimensions, non-uniform shrinkage can result in serious residual stress or cracking in parts. Such stress or cracking may preclude any salvage of the parts by extra machining.
Shrinkage problems are greatly exacerbated as the dimensions and/or complexity of the shape increase. A 1% variation in shrinkage means a one centimeter dimensional variation over a one meter length. Complexity of shape presents challenges in maintaining uniformity of powder packing and uniformity of temperature exposure in subsequent sintering. Control of packing and sintering conditions are often critical to obtaining accurate, uniform, and reproducible shrinkage.
While the amount of shrinkage during sintering is often lower for bodies in which more residual porosity is desired (e.g. for thermal insulation purposes), these bodies still present challenging sintering problems. The uniformity and character of the residual porosity sought plays a key role in the resultant property balance of the product. Thus, for example, a uniform distribution of spherical pores provides significant reduction in properties such as thermal conductivity, but among the least reductions in stiffness and strength for a given porosity level. Accurate control of the original powder compact characteristics and the sintering conditions are often essential to achieving a desired porosity configuration in combination with other desired properties. While potentially reducing the amount of shrinkage, residual porosity also can make the body more susceptible to shrinkage variations due to porosity variations in the body.